Embodiments of the invention relate generally to a system and method for actuating magnetic resonance (MR) coil elements and, more particularly, to a system and method for selectively activating and deactivating MR coil elements.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization,” MZ, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
An MR imaging apparatus typically includes a number of transmission coils and a number of receiving or reception coils to generate and receive the emitted signals. Generally, the radio-frequency transmission coil transmits at the Larmor frequency, resulting in an echo signal that is received by a reception coil and digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The reception coil is tuned to the Larmor frequency, allowing the reception coil to receive the echo signal. Because the reception coil is tuned to the Larmor frequency, the reception coil must be detuned during the transmission phase to prevent unwanted current from being induced within the reception coil from the transmitted magnetic field.
A detuning circuit, typically including a PIN diode, deactivates (i.e., detunes) the reception coil during operation of the transmission coil. The PIN diode is triggered via a current signal sent through a high-conductivity trace (typically silver or copper) from a drive unit to the PIN diode. Resistive elements, for example, discrete inductors and/or resistor and capacitor networks, are typically positioned along the length of the trace to dissipate the heat generated within the trace due to the large magnetic fields present in the MR environment. However, resistive elements increase the cost and design complexity of the detuning circuits. Further, hot spots may result at the locations where the discrete resistive elements are positioned on the traces that may lead to premature equipment failure or patient discomfort.
It is therefore desirable to provide a system and method for activating and deactivating an MR receiver coil element that minimizes the ‘hot spots’ caused by discrete power dissipation methods and decreases the cost and design complexity of an MR receiver coil apparatus.